Convert contrib modules to use the extension facility.

This isn't fully tested as yet, in particular I'm not sure that the
"foo--unpackaged--1.0.sql" scripts are OK.  But it's time to get some
buildfarm cycles on it.

sepgsql is not converted to an extension, mainly because it seems to
require a very nonstandard installation process.

Dimitri Fontaine and Tom Lane

contrib/intarray/sql/_int.sql

@@ -1,12 +1,4 @@
---
--- first, define the datatype.  Turn off echoing so that expected file
--- does not depend on contents of _int.sql.
---
-SET client_min_messages = warning;
-\set ECHO none
-\i _int.sql
-\set ECHO all
-RESET client_min_messages;
+CREATE EXTENSION intarray;
 
 SELECT intset(1234);
 SELECT icount('{1234234,234234}');